1. Field of Invention
The invention relates to an apparatus for improving the capabilities of existing reentry bodies (RBs). More specifically, the invention relates to retrofitting existing RBs with an increased fineness forward section and an aft section containing active control components such that the improved RB can correct for dispersive factors throughout the flight profile and is made independent of missile error sources and the RB deployment sequence. Conventional reentry bodies are, more accurately, ballistic reentry bodies (BRBs) in that they follow a trajectory determined by their release parameters. The reentry vehicles of the present invention permits conventional BRBs to have increased capability beyond the release parameters and are referred to, with the improvements, as simply RBs.
2. Description of Prior Art
Reentry bodies (RBs) carried aboard ballistic missiles are typically designed to achieve an accuracy goal with a minimum size and weight. Additionally, RBs are generally released sequentially from a maneuvering missile "bus" such that each RB has a target impact footprint constrained by that RB's position in the release sequence and whatever deployment errors occur in the release of preceeding RBs propagate to the release of subsequent RBs. Therefore, improvement in the performance and accuracy of RBs may be achieved by both passive and active design modification.
Passive design improvement of existing RBs to obtain significantly increased performance is achieved by retrofitting a new forward section to any existing RB such that fineness (the ratio of length to width) of the RB is greatly increased as well as static margin. Thus for minimal penalties in increased size and weight, greatly increased stability in the presence of aerodynamic and atmospheric dispersive factors is achieved.
Active design improvement of existing RBs presents a way of providing active control to the RBs to minimize the RB deployment errors resulting from release mechanisms, release sequence and hostile exo-atmospheric nuclear encounters. RB deployment accuracy is limited by the missile structural flexibility combined with the operation of the solid propellant gas generator control system. Since the gas generator must operate continuously once started, the on-off cycling with pulse width modulation of two thrust levels leads to continuing excitation of structure flexing and rigid body angular motion. This limits the pointing accuracy and generates RB tip-off rate residuals causing lateral velocity uncertainties. The explosive bolt separation upon release plus impulsive separation velocity and spin-up cause additional dispersive velocities. The RB traverse through the time modulated reaction control plumes of the missile further degrades the deployment accuracy due to relative position, attitude and velocity uncertainties between the RB and the missile bus as it maneuvers away and thrusts to the next RB drop. The mechanical misalignment between a RB and the missile bus propagates the separation velocity into the lateral direction. A larger separation velocity is desirable to minimize time spent subject to plume effects while a smaller separation velocity minimizes the effects of misalignment. The distribution of deployment velocity errors is highly asymmetric--the axial error being about twice the lateral error. To minimize the impact dispersion caused by the velocity errors, the RB is usually released in the null miss direction such that the axial component of velocity changes to trajectory is cancelled by flight time at the target. Thus the only deployment contribution to target impact error is the smaller lateral velocity error. However, the consequence of null miss pointing is that the RB reenters the atmosphere having a non-zero angle of attack with uncertainties caused by missile pointing, RB tip-off and spin-rate errors. The subsequent angle of attack convergence in the presence of these errors yields significant additional reentry dispersions when compared to zero angle of attack reentry. However, trade off of missile deployment error versus reentry error results in significant improvement using null miss pointing since missile deployment error predominates.
Several attempts have been made within the prior art to reduce error in RB deployment and thus improve accuracy. These include reduced missile bus thrust levels, increased settling down time to damp out rigid body rates and structural flexing, reduced separation velocity to reduce misalignment errors, plume avoidance maneuvers to keep the RB out of the bus plume during transit to the next drop, near nozzle off to minimize RB time in the plume during initial deployment, canted nozzles on the bus, and improvements in the bus control system sensors and electronics. These approaches, however, cause a significant increase in the duration of the deployment process which result in missile performance penalties as well as increasing vulnerability of the bus. The plume avoidance maneuvers significantly complicate the missile operation and the pre-set computations for fire control. RB loading arrangements, canted toward the center of the missile because of shroud clearance contraints, require complicated missile bus maneuvers to avoid RBs bumping into one another. The plume uncertainties during this process further reduce the effectiveness of the plume avoidance maneuvers. The gas dynamics of the time modulated plume are not well understood and are extremely difficult to determine empirically. The extended deployment time further exacerbates the velocity uncertainties between the guidance drop signal and the actual drop time. The mass properties and structural flexibility of the bus exhibit large variations as successive RBs are dropped making RBs deployed later in the sequence more sensitive to the various error sources. The present invention provides a way to avoid these limitations of the prior art by retrofitting a new forward and aft section to existing RBs to greatly improve their performance.